The present invention relates to field of printing and copying. More particularly, it relates to improvements in the structure of printing or image-transfer drums of the type having a resilient outer sleeve that are supported by an underlying mandrel. Such drums are used, for example, in electrostatic document printers and copiers for temporarily receiving a toner image from an image-recording element before there is re-transferred to an image-receiver sheet or the like.
In printing machines, copiers and the like, images are often formed on or transferred to a drum having a flexible or resilient outer sleeve that, from time to time, requires replacement. Typically, the sleeve is operatively supported by a metal cylinder or mandrel. In loading the sleeve onto the mandrel, it is common to inject air into the sleeve, thereby slightly expanding the sleeve diameter, while sliding the sleeve axially onto the mandrel""s supporting surface. Usually, the nominal diameter of the resilient sleeve is slightly less than the mandrel diameter. Thus, upon discontinuing the air flow, the sleeve contracts onto the mandrel and forms a tight, interference fit.
In drum assemblies of the type described above, it is known to machine air ports or holes in the sleeve-loading end of the mandrel. See, e.g., the disclosure of U.S. Pat. No. 4,119,032. Air supplied through the mandrel interior emerges through these air ports and provides an air bearing for expanding the sleeve. A uniform air bearing that is sufficient to expand a sleeve of relatively non-elastic material often necessitates a close and uniform spacing of the air ports in the mandrel surface. This need can be problematic from a manufacturing cost standpoint when the mandrel is of relatively large diameter and/or is made of a relatively hard substance.
In view of the foregoing discussion, an object of this invention is to provide a low-cost mandrel assembly for supporting a flexible sleeve of the type described.
According to one aspect of the invention, a mandrel for supporting a removable outer sleeve comprises: (a) a cylindrical tube of predetermined outer diameter; and (b) a circular end plate positioned at one end of the tube and operatively connected thereto. The end plate has a diameter substantially equal to the predetermined outer diameter of the cylindrical tube and has a plurality of radially-extending slots formed in a planar surface thereof that faces inwardly, toward the center of said tube. The slots are sufficiently long to extend from a centrally-located annular recess formed in the planar surface to the perimeter of the end plate. A pressure plate is operatively connected to the end plate and has a surface that cooperates with the slotted and recessed planar surface of the end plate to define a centrally-located air chamber and a plurality of radially-extending air passageways connecting the air chamber and the perimeter of the end plate. Upon connecting the air chamber to a source of pressurized air, a plurality of radially extending air streams are created at the periphery of the end plate. According to a preferred embodiment, both the end plate and said pressure plate are made of plastic, whereby the cylindrical tube and a supporting axle, both of which are made of metal, can be electrically isolated. This is particularly useful when the drum surface is to be electrically biased relative to a machine frame that supports the axle. Also preferred is that the peripheral edge of the end plate is tapered to facilitate the loading of the removable sleeve onto said mandrel.
By virtue of having the air ports of the air bearing defined by a pair of confronting plates, each preferably being molded from plastic, the above-noted machining process of prior art mandrel is obviated. Thus, the manufacturing costs are significantly reduced.
The invention and its advantages will be better understood from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings in which like reference characters denote like parts.